Executing a gather operation on a parallel computer that includes a plurality of compute nodes

ABSTRACT

Executing a gather operation on a parallel computer that includes a plurality of compute nodes, including: dividing, by each task in an operational group of tasks, a send buffer containing contribution data into a plurality of chunks of data, each chunk of data located at an offset within the send buffer; sending, by each task in the operational group of tasks, one chunk of data to a root task through a data communications thread for each chunk of data; receiving the chunks of data by the root task; and storing, by the root task, each chunk of data in a receive buffer of the root task in dependence upon the offset of each chunk of data within the send buffer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is data processing, or, more specifically, methods, apparatus, and products for executing a gather operation on a parallel computer that includes a plurality of compute nodes.

2. Description of Related Art

The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely complicated devices. Today's computers are much more sophisticated than early systems such as the EDVAC. Computer systems typically include a combination of hardware and software components, application programs, operating systems, processors, buses, memory, input/output devices, and so on. As advances in semiconductor processing and computer architecture push the performance of the computer higher and higher, more sophisticated computer software has evolved to take advantage of the higher performance of the hardware, resulting in computer systems today that are much more powerful than just a few years ago.

Modern computing systems can be embodied as parallel computers that include a large number of compute nodes. Such parallel computers can execute parallel operations that require communication between many of the compute nodes in the parallel computer. Such compute nodes may communicate using serial data communications, where one message is sent after another between the compute nodes. Such serial communications can present a bottleneck in the execution of such parallel applications.

SUMMARY OF THE INVENTION

Methods, apparatuses, and products for executing a gather operation on a parallel computer that includes a plurality of compute nodes, including: dividing, by each task in an operational group of tasks, a send buffer containing contribution data into a plurality of chunks of data, each chunk of data located at an offset within the send buffer; sending, by each task in the operational group of tasks, one chunk of data to a root task through a data communications thread for each chunk of data; receiving the chunks of data by the root task; and storing, by the root task, each chunk of data in a receive buffer of the root task in dependence upon the offset of each chunk of data within the send buffer.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular descriptions of exemplary embodiments of the invention as illustrated in the accompanying drawings wherein like reference numbers generally represent like parts of exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a parallel computer for executing a gather operation according to embodiments of the present invention.

FIG. 2 sets forth a block diagram of an example compute node useful in a parallel computer capable of executing a gather operation according to embodiments of the present invention.

FIG. 3 sets forth a block diagram of an example Point-To-Point Adapter useful in systems for executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention.

FIG. 4 sets forth a block diagram of an example Global Combining Network Adapter useful in systems for executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention.

FIG. 5 sets forth a line drawing illustrating an example data communications network optimized for point-to-point operations useful in systems capable of executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention.

FIG. 6 sets forth a line drawing illustrating an example global combining network useful in systems capable of executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention.

FIG. 7 sets forth a flow chart illustrating an example method for executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention.

FIG. 8 sets forth a flow chart illustrating an example method for executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Example methods, apparatuses, and products for executing a gather operation on a parallel computer that includes a plurality of compute nodes in accordance with the present invention are described with reference to the accompanying drawings, beginning with FIG. 1. FIG. 1 illustrates a parallel computer (100) for executing a gather operation according to embodiments of the present invention. The system of FIG. 1 includes a parallel computer (100), non-volatile memory for the computer in the form of a data storage device (118), an output device for the computer in the form of a printer (120), and an input/output device for the computer in the form of a computer terminal (122).

The parallel computer (100) in the example of FIG. 1 includes a plurality of compute nodes (102). The compute nodes (102) are coupled for data communications by several independent data communications networks including a high speed Ethernet network (174), a Joint Test Action Group (‘JTAG’) network (104), a global combining network (106) which is optimized for collective operations using a binary tree network topology, and a point-to-point network (108), which is optimized for point-to-point operations using a torus network topology. The global combining network (106) is a data communications network that includes data communications links connected to the compute nodes (102) so as to organize the compute nodes (102) as a binary tree. Each data communications network is implemented with data communications links among the compute nodes (102). The data communications links provide data communications for parallel operations among the compute nodes (102) of the parallel computer (100).

The compute nodes (102) of the parallel computer (100) are organized into at least one operational group (132) of compute nodes for collective parallel operations on the parallel computer (100). Each operational group (132) of compute nodes is the set of compute nodes upon which a collective parallel operation executes. Each compute node in the operational group (132) is assigned a unique rank that identifies the particular compute node in the operational group (132). Collective operations are implemented with data communications among the compute nodes of an operational group. Collective operations are those functions that involve all the compute nodes of an operational group (132). A collective operation is an operation, a message-passing computer program instruction that is executed simultaneously, that is, at approximately the same time, by all the compute nodes in an operational group (132) of compute nodes. Such an operational group (132) may include all the compute nodes (102) in a parallel computer (100) or a subset all the compute nodes (102). Collective operations are often built around point-to-point operations. A collective operation requires that all processes on all compute nodes within an operational group (132) call the same collective operation with matching arguments. A ‘broadcast’ is an example of a collective operation for moving data among compute nodes of an operational group. A ‘reduce’ operation is an example of a collective operation that executes arithmetic or logical functions on data distributed among the compute nodes of an operational group (132). An operational group (132) may be implemented as, for example, an MPI ‘communicator.’

‘MPI’ refers to ‘Message Passing Interface,’ a prior art parallel communications library, a module of computer program instructions for data communications on parallel computers. Examples of prior-art parallel communications libraries that may be improved for use in systems configured according to embodiments of the present invention include MPI and the ‘Parallel Virtual Machine’ (‘PVM’) library. PVM was developed by the University of Tennessee, The Oak Ridge National Laboratory and Emory University. MPI is promulgated by the MPI Forum, an open group with representatives from many organizations that define and maintain the MPI standard. MPI at the time of this writing is a de facto standard for communication among compute nodes running a parallel program on a distributed memory parallel computer. This specification sometimes uses MPI terminology for ease of explanation, although the use of MPI as such is not a requirement or limitation of the present invention.

Some collective operations have a single originating or receiving process running on a particular compute node in an operational group (132). For example, in a ‘broadcast’ collective operation, the process on the compute node that distributes the data to all the other compute nodes is an originating process. In a ‘gather’ operation, for example, the process on the compute node that received all the data from the other compute nodes is a receiving process. The compute node on which such an originating or receiving process runs is referred to as a logical root.

Most collective operations are variations or combinations of four basic operations: broadcast, gather, scatter, and reduce. The interfaces for these collective operations are defined in the MPI standards promulgated by the MPI Forum. Algorithms for executing collective operations, however, are not defined in the MPI standards. In a broadcast operation, all processes specify the same root process, whose buffer contents will be sent. Processes other than the root specify receive buffers. After the operation, all buffers contain the message from the root process.

A scatter operation, like the broadcast operation, is also a one-to-many collective operation. In a scatter operation, the logical root divides data on the root into segments and distributes a different segment to each compute node in the operational group (132). In scatter operation, all processes typically specify the same receive count. The send arguments are only significant to the root process, whose buffer actually contains sendcount*N elements of a given datatype, where N is the number of processes in the given group of compute nodes. The send buffer is divided and dispersed to all processes (including the process on the logical root). Each compute node is assigned a sequential identifier termed a ‘rank.’ After the operation, the root has sent sendcount data elements to each process in increasing rank order. Rank 0 receives the first sendcount data elements from the send buffer. Rank 1 receives the second sendcount data elements from the send buffer, and so on.

A gather operation is a many-to-one collective operation that is a complete reverse of the description of the scatter operation. That is, a gather is a many-to-one collective operation in which elements of a datatype are gathered from the ranked compute nodes into a receive buffer in a root node.

A reduction operation is also a many-to-one collective operation that includes an arithmetic or logical function performed on two data elements. All processes specify the same ‘count’ and the same arithmetic or logical function. After the reduction, all processes have sent count data elements from compute node send buffers to the root process. In a reduction operation, data elements from corresponding send buffer locations are combined pair-wise by arithmetic or logical operations to yield a single corresponding element in the root process' receive buffer. Application specific reduction operations can be defined at runtime. Parallel communications libraries may support predefined operations. MPI, for example, provides the following predefined reduction operations:

-   -   MPI_MAX maximum     -   MPI_MIN minimum     -   MPI_SUM sum     -   MPI_PROD product     -   MPI_LAND logical and     -   MPI_BAND bitwise and     -   MPI_LOR logical or     -   MPI_BOR bitwise or     -   MPI_LXOR logical exclusive or     -   MPI_BXOR bitwise exclusive or

In addition to compute nodes, the parallel computer (100) includes input/output (‘I/O’) nodes (110, 114) coupled to compute nodes (102) through the global combining network (106). The compute nodes (102) in the parallel computer (100) may be partitioned into processing sets such that each compute node in a processing set is connected for data communications to the same I/O node. Each processing set, therefore, is composed of one I/O node and a subset of compute nodes (102). The ratio between the number of compute nodes to the number of I/O nodes in the entire system typically depends on the hardware configuration for the parallel computer (102). For example, in some configurations, each processing set may be composed of eight compute nodes and one I/O node. In some other configurations, each processing set may be composed of sixty-four compute nodes and one I/O node. Such example are for explanation only, however, and not for limitation. Each I/O node provides I/O services between compute nodes (102) of its processing set and a set of I/O devices. In the example of FIG. 1, the I/O nodes (110, 114) are connected for data communications I/O devices (118, 120, 122) through local area network (‘LAN’) (130) implemented using high-speed Ethernet.

The parallel computer (100) of FIG. 1 also includes a service node (116) coupled to the compute nodes through one of the networks (104). Service node (116) provides services common to pluralities of compute nodes, administering the configuration of compute nodes, loading programs into the compute nodes, starting program execution on the compute nodes, retrieving results of program operations on the compute nodes, and so on. Service node (116) runs a service application (124) and communicates with users (128) through a service application interface (126) that runs on computer terminal (122).

The parallel computer (100) of FIG. 1 operates generally for executing a gather operation in accordance with embodiments of the present invention. As mentioned above, some parallel application may be split into parallel processes or parallel tasks. For simplicity in this specification a single compute node is often described as executing a single task. Readers will understand however that a compute node may execute in any number of tasks. In the example of FIG. 1, each compute node (102) may execute a number of tasks where at least one of the compute nodes executes a number of tasks that is different than the number of tasks executed by another one of the compute nodes.

Executing a gather operation according to embodiments of the present invention is generally implemented on a parallel computer that includes a plurality of compute nodes organized for collective operations through at least one data communications network. In fact, such computers may include thousands of such compute nodes. Each compute node is in turn itself a kind of computer composed of one or more computer processing cores, its own computer memory, and its own input/output adapters. For further explanation, therefore, FIG. 2 sets forth a block diagram of an example compute node (102) useful in a parallel computer capable of executing a gather operation according to embodiments of the present invention. The compute node (102) of FIG. 2 includes a plurality of processing cores (165) as well as RAM (156). The processing cores (165) of FIG. 2 may be configured on one or more integrated circuit dies. Processing cores (165) are connected to RAM (156) through a high-speed memory bus (155) and through a bus adapter (194) and an extension bus (168) to other components of the compute node. Stored in RAM (156) is an application program (159), a module of computer program instructions that carries out parallel, user-level data processing using parallel algorithms.

Also stored RAM (156) is a parallel communications library (161), a library of computer program instructions that carry out parallel communications among compute nodes, including point-to-point operations as well as collective operations. A library of parallel communications routines may be developed from scratch for use in systems according to embodiments of the present invention, using a traditional programming language such as the C programming language, and using traditional programming methods to write parallel communications routines that send and receive data among nodes on two independent data communications networks. Alternatively, existing prior art libraries may be improved to operate according to embodiments of the present invention. Examples of prior-art parallel communications libraries include the ‘Message Passing Interface’ (‘MPI’) library and the ‘Parallel Virtual Machine’ (‘PVM’) library.

Also stored in RAM (156) is an operating system (162), a module of computer program instructions and routines for an application program's access to other resources of the compute node. It is typical for an application program and parallel communications library in a compute node of a parallel computer to run a single thread of execution with no user login and no security issues because the thread is entitled to complete access to all resources of the node. The quantity and complexity of tasks to be performed by an operating system on a compute node in a parallel computer therefore are smaller and less complex than those of an operating system on a serial computer with many threads running simultaneously. In addition, there is no video I/O on the compute node (102) of FIG. 2, another factor that decreases the demands on the operating system. The operating system (162) may therefore be quite lightweight by comparison with operating systems of general purpose computers, a pared down version as it were, or an operating system developed specifically for operations on a particular parallel computer. Operating systems that may usefully be improved, simplified, for use in a compute node include UNIX™, Linux™, Windows XP™, AIX™, IBM's i5/OS™, and others as will occur to those of skill in the art.

The example compute node (102) of FIG. 2 includes several communications adapters (172, 176, 180, 188) for implementing data communications with other nodes of a parallel computer. Such data communications may be carried out serially through RS-232 connections, through external buses such as USB, through data communications networks such as IP networks, and in other ways as will occur to those of skill in the art. Communications adapters implement the hardware level of data communications through which one computer sends data communications to another computer, directly or through a network. Examples of communications adapters useful in apparatus useful for executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention include modems for wired communications, Ethernet (IEEE 802.3) adapters for wired network communications, and 802.11b adapters for wireless network communications.

The data communications adapters in the example of FIG. 2 include a Gigabit Ethernet adapter (172) that couples example compute node (102) for data communications to a Gigabit Ethernet (174). Gigabit Ethernet is a network transmission standard, defined in the IEEE 802.3 standard, that provides a data rate of 1 billion bits per second (one gigabit). Gigabit Ethernet is a variant of Ethernet that operates over multimode fiber optic cable, single mode fiber optic cable, or unshielded twisted pair.

The data communications adapters in the example of FIG. 2 include a JTAG Slave circuit (176) that couples example compute node (102) for data communications to a JTAG Master circuit (178). JTAG is the usual name used for the IEEE 1149.1 standard entitled Standard Test Access Port and Boundary-Scan Architecture for test access ports used for testing printed circuit boards using boundary scan. JTAG is so widely adapted that, at this time, boundary scan is more or less synonymous with JTAG. JTAG is used not only for printed circuit boards, but also for conducting boundary scans of integrated circuits, and is also useful as a mechanism for debugging embedded systems, providing a convenient alternative access point into the system. The example compute node of FIG. 2 may be all three of these: It typically includes one or more integrated circuits installed on a printed circuit board and may be implemented as an embedded system having its own processing core, its own memory, and its own I/O capability. JTAG boundary scans through JTAG Slave (176) may efficiently configure processing core registers and memory in compute node (102) for use in dynamically reassigning a connected node to a block of compute nodes useful in systems for executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention.

The data communications adapters in the example of FIG. 2 include a Point-To-Point Network Adapter (180) that couples example compute node (102) for data communications to a network (108) that is optimal for point-to-point message passing operations such as, for example, a network configured as a three-dimensional torus or mesh. The Point-To-Point Adapter (180) provides data communications in six directions on three communications axes, x, y, and z, through six bidirectional links: +x (181), −x (182), +y (183), −y (184), +z (185), and −z (186).

The data communications adapters in the example of FIG. 2 include a Global Combining Network Adapter (188) that couples example compute node (102) for data communications to a global combining network (106) that is optimal for collective message passing operations such as, for example, a network configured as a binary tree. The Global Combining Network Adapter (188) provides data communications through three bidirectional links for each global combining network (106) that the Global Combining Network Adapter (188) supports. In the example of FIG. 2, the Global Combining Network Adapter (188) provides data communications through three bidirectional links for global combining network (106): two to children nodes (190) and one to a parent node (192).

The example compute node (102) includes multiple arithmetic logic units (‘ALUs’). Each processing core (165) includes an ALU (166), and a separate ALU (170) is dedicated to the exclusive use of the Global Combining Network Adapter (188) for use in performing the arithmetic and logical functions of reduction operations, including an allreduce operation. Computer program instructions of a reduction routine in a parallel communications library (161) may latch an instruction for an arithmetic or logical function into an instruction register (169). When the arithmetic or logical function of a reduction operation is a ‘sum’ or a ‘logical OR,’ for example, the collective operations adapter (188) may execute the arithmetic or logical operation by use of the ALU (166) in the processing core (165) or, typically much faster, by use of the dedicated ALU (170) using data provided by the nodes (190, 192) on the global combining network (106) and data provided by processing cores (165) on the compute node (102).

Often when performing arithmetic operations in the global combining network adapter (188), however, the global combining network adapter (188) only serves to combine data received from the children nodes (190) and pass the result up the network (106) to the parent node (192). Similarly, the global combining network adapter (188) may only serve to transmit data received from the parent node (192) and pass the data down the network (106) to the children nodes (190). That is, none of the processing cores (165) on the compute node (102) contribute data that alters the output of ALU (170), which is then passed up or down the global combining network (106). Because the ALU (170) typically does not output any data onto the network (106) until the ALU (170) receives input from one of the processing cores (165), a processing core (165) may inject the identity element into the dedicated ALU (170) for the particular arithmetic operation being perform in the ALU (170) in order to prevent alteration of the output of the ALU (170). Injecting the identity element into the ALU, however, often consumes numerous processing cycles. To further enhance performance in such cases, the example compute node (102) includes dedicated hardware (171) for injecting identity elements into the ALU (170) to reduce the amount of processing core resources required to prevent alteration of the ALU output. The dedicated hardware (171) injects an identity element that corresponds to the particular arithmetic operation performed by the ALU. For example, when the global combining network adapter (188) performs a bitwise OR on the data received from the children nodes (190), dedicated hardware (171) may inject zeros into the ALU (170) to improve performance throughout the global combining network (106).

For further explanation, FIG. 3 sets forth a block diagram of an example Point-To-Point Adapter (180) useful in systems for executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention. The Point-To-Point Adapter (180) is designed for use in a data communications network optimized for point-to-point operations, a network that organizes compute nodes in a three-dimensional torus or mesh. The Point-To-Point Adapter (180) in the example of FIG. 3 provides data communication along an x-axis through four unidirectional data communications links, to and from the next node in the −x direction (182) and to and from the next node in the +x direction (181). The Point-To-Point Adapter (180) of FIG. 3 also provides data communication along a y-axis through four unidirectional data communications links, to and from the next node in the −y direction (184) and to and from the next node in the +y direction (183). The Point-To-Point Adapter (180) of FIG. 3 also provides data communication along a z-axis through four unidirectional data communications links, to and from the next node in the −z direction (186) and to and from the next node in the +z direction (185).

For further explanation, FIG. 4 sets forth a block diagram of an example Global Combining Network Adapter (188) useful in systems for executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention. The Global Combining Network Adapter (188) is designed for use in a network optimized for collective operations, a network that organizes compute nodes of a parallel computer in a binary tree. The Global Combining Network Adapter (188) in the example of FIG. 4 provides data communication to and from children nodes of a global combining network through four unidirectional data communications links (190), and also provides data communication to and from a parent node of the global combining network through two unidirectional data communications links (192).

For further explanation, FIG. 5 sets forth a line drawing illustrating an example data communications network (108) optimized for point-to-point operations useful in systems capable of executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention. In the example of FIG. 5, dots represent compute nodes (102) of a parallel computer, and the dotted lines between the dots represent data communications links (103) between compute nodes. The data communications links are implemented with point-to-point data communications adapters similar to the one illustrated for example in FIG. 3, with data communications links on three axis, x, y, and z, and to and from in six directions +x (181), −x (182), +y (183), −y (184), +z (185), and −z (186). The links and compute nodes are organized by this data communications network optimized for point-to-point operations into a three dimensional mesh (105). The mesh (105) has wrap-around links on each axis that connect the outermost compute nodes in the mesh (105) on opposite sides of the mesh (105). These wrap-around links form a torus (107). Each compute node in the torus has a location in the torus that is uniquely specified by a set of x, y, z coordinates. Readers will note that the wrap-around links in the y and z directions have been omitted for clarity, but are configured in a similar manner to the wrap-around link illustrated in the x direction. For clarity of explanation, the data communications network of FIG. 5 is illustrated with only 27 compute nodes, but readers will recognize that a data communications network optimized for point-to-point operations for use in executing a gather operation on a parallel computer that includes a plurality of compute nodes in accordance with embodiments of the present invention may contain only a few compute nodes or may contain thousands of compute nodes. For ease of explanation, the data communications network of FIG. 5 is illustrated with only three dimensions, but readers will recognize that a data communications network optimized for point-to-point operations for use in executing a gather operation on a parallel computer that includes a plurality of compute nodes in accordance with embodiments of the present invention may in fact be implemented in two dimensions, four dimensions, five dimensions, and so on. Several supercomputers now use five dimensional mesh or torus networks, including, for example, IBM's Blue Gene Q™.

For further explanation, FIG. 6 sets forth a line drawing illustrating an example global combining network (106) useful in systems capable of executing a gather operation on a parallel computer that includes a plurality of compute nodes according to embodiments of the present invention. The example data communications network of FIG. 6 includes data communications links (103) connected to the compute nodes so as to organize the compute nodes as a tree. In the example of FIG. 6, dots represent compute nodes (102) of a parallel computer, and the dotted lines (103) between the dots represent data communications links between compute nodes. The data communications links are implemented with global combining network adapters similar to the one illustrated for example in FIG. 4, with each node typically providing data communications to and from two children nodes and data communications to and from a parent node, with some exceptions. Nodes in the global combining network (106) may be characterized as a physical root node (202), branch nodes (204), and leaf nodes (206). The physical root (202) has two children but no parent and is so called because the physical root node (202) is the node physically configured at the top of the binary tree. The leaf nodes (206) each has a parent, but leaf nodes have no children. The branch nodes (204) each has both a parent and two children. The links and compute nodes are thereby organized by this data communications network optimized for collective operations into a binary tree (106). For clarity of explanation, the data communications network of FIG. 6 is illustrated with only 31 compute nodes, but readers will recognize that a global combining network (106) optimized for collective operations for use in executing a gather operation on a parallel computer that includes a plurality of compute nodes in accordance with embodiments of the present invention may contain only a few compute nodes or may contain thousands of compute nodes.

In the example of FIG. 6, each node in the tree is assigned a unit identifier referred to as a ‘rank’ (250). The rank actually identifies a task or process that is executing a parallel operation according to embodiments of the present invention. Using the rank to identify a node assumes that only one such task is executing on each node. To the extent that more than one participating task executes on a single node, the rank identifies the task as such rather than the node. A rank uniquely identifies a task's location in the tree network for use in both point-to-point and collective operations in the tree network. The ranks in this example are assigned as integers beginning with 0 assigned to the root tasks or root node (202), 1 assigned to the first node in the second layer of the tree, 2 assigned to the second node in the second layer of the tree, 3 assigned to the first node in the third layer of the tree, 4 assigned to the second node in the third layer of the tree, and so on. For ease of illustration, only the ranks of the first three layers of the tree are shown here, but all compute nodes in the tree network are assigned a unique rank.

For further explanation, FIG. 7 sets forth a flow chart illustrating an example method for executing a gather operation on a parallel computer (702) that includes a plurality of compute nodes (704, 736) according to embodiments of the present invention. As described above, a gather operation is a many-to-one collective operation in which elements of a datatype are gathered from the ranked compute nodes, such as compute node (704), into a receive buffer (734) in a root compute node (736). Although not depicted in FIG. 7, readers will appreciate that elements of a datatype may be gathered from many compute nodes and stored into a receive buffer (734) in a root compute node (736).

The example method depicted in FIG. 7 includes dividing (712), by each task (710) in an operational group of tasks, a send buffer (706) containing contribution data (708) into a plurality of chunks (722, 724, 726) of data. In the example method of FIG. 7, the send buffer (706) can be embodied as computer memory in a compute node (704) for storing data to be sent to another compute node (736). Such data can include, for example, the contribution data (708) of FIG. 7. The contribution data (708) of FIG. 7 represents data that is to be used during the execution of a collective operation. In such an example, the contribution data (708) represents one compute node's contribution to the collective operation. In the example method of FIG. 7, a send buffer (706) containing contribution data (708) may be divided (712), for example, into a plurality of chunks (722, 724, 726) of data that are off a size such that each chuck may be transmitted to the root compute node (736) in a single message.

In the example method of FIG. 7, each chunk (722, 724, 726) of data is located at an offset within the send buffer (706). Consider an example in which each chunk (722, 724, 726) of data in FIG. 7 is 1 MB in size and the contribution data (708) is 3 MB in size. In such an example, a first chunk (722) of data may be located at an offset of 0 within the send buffer (706), such that the first chunk (722) of data represents the first MB of the contribution data (708). Likewise, a second chunk (724) of data may be located at an offset of 1 MB within the send buffer (706), such that the second chunk (724) of data represents the second MB of the contribution data (708). In addition, a third chunk (726) of data may be located at an offset of 2 MB within the send buffer (706), such that the third chunk (726) of data represents the third MB of the contribution data (708). In such a way, the offset within the send buffer (706) represents the location within the send buffer (706) of a particular chunk (722, 724, 726) of data.

The example method depicted in FIG. 7 also includes sending (714), by each task (710) in the operational group of tasks, one chunk (722, 724, 726) of data to a root task (728) through a data communications thread (716, 718, 720) for each chunk (722, 724, 726) of data. In the example method of FIG. 7, a data communications thread (716, 718, 720) may be created for each chunk (722, 724, 726) of data, such that each chunk (722, 724, 726) of data may be transmitted in parallel. That is, a first data communications thread (716) may transmit a first chunk (722) of data, a second data communications thread (718) may transmit a second chunk (724) of data, and a third data communications thread (720) may transmit a third chunk (726) of data—with each transmission occurring simultaneously. In such an example, the number of chunks (722, 724, 726) of data that need to be sent to the root task (728) may be identified and a data communications thread (716, 718, 720) may be instantiated for each chunk (722, 724, 726) of data.

The example method depicted in FIG. 7 also includes receiving (730) the chunks (722, 724, 726) of data by the root task (728). In the example method of FIG. 7, each chunk (722, 724, 726) of data may be received (730) by the root task (728) over any of the data communications networks described above with reference to FIGS. 1-6. Upon receiving (730) the chunks (722, 724, 726) of data by the root task (728), the root task (728) may attempt to reassemble the chunks (722, 724, 726) of data so as to recreate the contribution data (708) that was divided into the chunks (722, 724, 726) of data.

The example method depicted in FIG. 7 also includes storing (732), by the root task (728), each chunk (722, 724, 726) of data in a receive buffer (734) of the root task (728) in dependence upon the offset of each chunk of data within the send buffer. Storing (732) each chunk (722, 724, 726) of data in a receive buffer (734) of the root task (728) in dependence upon the offset of each chunk of data within the send buffer may be carried out, for example, by using the offset of each chunk of data within the send buffer (706) to order the chunks (722, 724, 726) of data received by the root task (728) in such a way so as to replicate the contribution data (708) in the receive buffer (734). Consider the example described above in which each chunk (722, 724, 726) of data in FIG. 7 is 1 MB in size and the contribution data (708) is 3 MB in size, where the first chunk (722) of data is located at an offset of 0 within the send buffer (706), the second chunk (724) of data is located at an offset of 1 MB within the send buffer (706), and the third chunk (726) of data is located at an offset of 2 MB within the send buffer (706). In such an example, storing (732) each chunk (722, 724, 726) of data in the receive buffer (734) of the root task (728) in dependence upon the offset of each chunk of data within the send buffer may be carried out by the first chunk (722) of data at an offset of 0 within the receive buffer (734), storing the second chunk (724) of data at an offset of 1 MB within the receive buffer (734), and storing the third chunk (726) of data at an offset of 2 MB within the receive buffer (734).

For further explanation, FIG. 8 sets forth a flow chart illustrating an additional example method for executing a gather operation on a parallel computer (702) that includes a plurality of compute nodes (704, 736) according to embodiments of the present invention. The example method depicted in FIG. 8 is similar to the example method depicted in FIG. 7, as it also includes dividing (712) a send buffer (706) containing contribution data (708) into a plurality of chunks (722, 724, 726) of data, sending (714) one chunk (722, 724, 726) of data to a root task (728) through a data communications thread (716, 718, 720) for each chunk (722, 724, 726) of data, receiving (730) the chunks (722, 724, 726) of data by the root task (728), and storing (732) each chunk (722, 724, 726) of data in a receive buffer (734) of the root task (728).

In the example method depicted in FIG. 8, dividing (712) a send buffer (706) containing contribution data (708) into a plurality of chunks (722, 724, 726) of data can include dividing (802) the send buffer (706) into the plurality of chunks (722, 724, 726) of data in dependence upon a message size. In the example method of FIG. 8, the message size may represent the size of messages that are received (730) by root task (728) via the data communications thread (716, 718, 720). In such an example, dividing (802) the send buffer (706) into the plurality of chunks (722, 724, 726) of data in dependence upon a message size may be carried out by dividing the contribution data (708) into enough chunks (722, 724, 726) of data such that each of the data communications threads (716, 718, 720) only needs to send a single message to the root task (728). Consider an example in which the contribution data (708) is 10 MB in size and the root task (728) receives messages that are 1 MB in size via the each of the data communications threads (716, 718, 720). In such an example, the send buffer (706) may be divided into ten chunks (722, 724, 726) of data that are each 1 MB in size, such that ten data communications threads (716, 718, 720) may be created and ten messages may be sent to the root task (728) in parallel, thereby only requiring one message to be sent to the root task (728) by each of the ten data communications threads (716, 718, 720). In such an example, transferring the contribution data (708) to the root task (728) may be done without requiring multiple rounds of data transfers from the task (710) that are carried out serially.

In the example method depicted in FIG. 8, dividing (712) a send buffer (706) containing contribution data (708) into a plurality of chunks (722, 724, 726) of data can alternatively include dividing (803) the send buffer (706) into the plurality of chunks (722, 724, 726) of data in dependence upon a predetermined data type. In the example method of FIG. 8, when tasks (710, 728) in a parallel computer (702) are participating in a gather operation, the tasks (710, 728) may communicate using predetermined data types. That is, functions such as an MPI gather require that type of data that is passed between the participating tasks be of a predetermined data type. Such a predetermined data type can include standard MPI data types, such as an integer, a character, and so on). Alternatively, such a predetermined data type can include a user-defined data type created, for example, through the use of a structure definition. As such, dividing (803) the send buffer (706) into the plurality of chunks (722, 724, 726) of data in dependence upon a predetermined data type may be carried out by determining the type of data contained in the contribution data (708) and dividing the contribution data (708) into individual instances of each data type. For example, if the contribution data (708) is of an integer data type, dividing (803) the send buffer (706) into the plurality of chunks (722, 724, 726) of data in dependence upon a predetermined data type may be carried out by dividing the contribution data (708) into individual integers that collectively form the contribution data (708).

The example method depicted in FIG. 8 also includes creating (804), for each chunk (722, 724, 726) of data, a data communications thread (716, 718, 720). In the example method depicted in FIG. 8, an operational group of tasks may be implemented as an MPI subcommunicator, where the MPI subcommunicator is a subcommunicator of the MPI_COMM_WORLD communicator. In such an example, each data communications thread (716, 718, 720) may be implemented as a task of a separate MPI subcommunicator. That is, each data communications thread (716, 718, 720) may be implemented as a task of an MPI subcommunicator that is separate and distinct from the MPI subcommunicator that supports the operational group of tasks. In such an example, creating (804) a data communications thread (716, 718, 720) for each chunk (722, 724, 726) of data may therefore be carried out by spawning subcommunicators from the MPI subcommunicator that supports the operational group of tasks, for example, through the use of the MPI_COMM_SPLIT function.

In the example method depicted in FIG. 8, storing (732) each chunk (722, 724, 726) of data in a receive buffer (734) of the root task (728) can include storing (806), in the receive buffer (734) of the root task (728), each chunk (722, 724, 726) of data in the same order as the chucks of data were stored in the send buffer (706). In the example method depicted in FIG. 8, storing (806) each chunk (722, 724, 726) of data in the same order as the chucks of data were stored in the send buffer (706) may be carried out by using the offset of each chunk of data within the send buffer (706) to order the chunks (722, 724, 726) of data received by the root task (728), such that the ordering of the chunks (722, 724, 726) of data is also maintained in the receive buffer (734) of the root task (728). Consider an example in which each chunk (722, 724, 726) of data in FIG. 8 is embodied as an integer and the contribution data (708) is embodied as an array of ten integers. In such an example, storing (806), in the receive buffer (734) of the root task (728), each chunk (722, 724, 726) of data in the same order as the chucks of data were stored in the send buffer (706) may be carried out by be receiving the index into the array maintained in the send buffer (706) for each chuck (724, 726, 728) of data that is received by the root task (728) and storing each chuck (724, 726, 728) of data that is received by the root task (728) in the identical location of an array of ten integers in the receive buffer (734) of the root task (728).

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device.

A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

Aspects of the present invention are described above with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

It will be understood from the foregoing description that modifications and changes may be made in various embodiments of the present invention without departing from its true spirit. The descriptions in this specification are for purposes of illustration only and are not to be construed in a limiting sense. The scope of the present invention is limited only by the language of the following claims. 

What is claimed is:
 1. An apparatus for executing a gather operation on a parallel computer that includes a plurality of compute nodes, the apparatus comprising a computer processor, a computer memory operatively coupled to the computer processor, the computer memory having disposed within it computer program instructions that, when executed by the computer processor, cause the apparatus to carry out the steps of: dividing, by each task in an operational group of tasks of a compute node, a send buffer containing contribution data into a plurality of chunks of data, each chunk of data located at an offset within the send buffer; sending, by each task in the operational group of tasks, one chunk of data to a root task through a data communications thread for each chunk of data including instantiating a separate data communication thread for each chunk of data, wherein each chunk of data is sent in parallel from the compute node to the root task; receiving the chunks of data by the root task; and storing, by the root task, each chunk of data in a receive buffer of the root task in dependence upon the offset of each chunk of data within the send buffer; wherein dividing the send buffer containing contribution data into a plurality of chunks of data further comprises dividing the send buffer into the plurality of chunks of data in dependence upon a predetermined data type.
 2. The apparatus of claim 1 further comprising computer program instructions that, when executed by the computer processor, cause the apparatus to carry out the step of creating, for each chunk of data, a data communications thread.
 3. The apparatus of claim 1 wherein dividing the send buffer containing contribution data into the plurality of chunks of data further comprises dividing the send buffer into the plurality of chunks of data in dependence upon a message size.
 4. The apparatus of claim 1 wherein storing each chunk of data in the receive buffer of the root task in dependence upon the offset of each chunk of data within the send buffer further comprises storing, in the receive buffer of the root task, each chunk of data in the same order as the chucks of data were stored in the send buffer.
 5. The apparatus of claim 1 wherein the operational group of tasks are implemented as a Message Passing Interface (‘MPI’) subcommunicator and each data communications thread is implemented as a task of a separate MPI subcommunicator.
 6. A computer program product for executing a gather operation on a parallel computer that includes a plurality of compute nodes, the computer program product disposed upon a computer readable storage medium, wherein the computer readable storage medium is not a signal, the computer program product comprising computer program instructions that, when executed, cause a computer to carry out the steps of: dividing, by each task in an operational group of tasks of a compute node, a send buffer containing contribution data into a plurality of chunks of data, each chunk of data located at an offset within the send buffer; sending, by each task in the operational group of tasks, one chunk of data to a root task through a data communications thread for each chunk of data including instantiating a separate data communication thread for each chunk of data, wherein each chunk of data is sent in parallel from the compute node to the root task; receiving the chunks of data by the root task; and storing, by the root task, each chunk of data in a receive buffer of the root task in dependence upon the offset of each chunk of data within the send buffer; wherein dividing the send buffer containing contribution data into a plurality of chunks of data further comprises dividing the send buffer into the plurality of chunks of data in dependence upon a predetermined data type.
 7. The computer program product of claim 6 further comprising computer program instructions that, when executed, cause the computer to carry out the steps of creating, for each chunk of data, a data communications thread.
 8. The computer program product of claim 6 wherein dividing the send buffer containing contribution data into the plurality of chunks of data further comprises dividing the send buffer into the plurality of chunks of data in dependence upon a message size.
 9. The computer program product of claim 6 wherein storing each chunk of data in the receive buffer of the root task in dependence upon the offset of each chunk of data within the send buffer further comprises storing, in the receive buffer of the root task, each chunk of data in the same order as the chucks of data were stored in the send buffer.
 10. The computer program product of claim 6 wherein the operational group of tasks are implemented as a Message Passing Interface (‘MPI’) subcommunicator and each data communications thread is implemented as a task of a separate MPI subcommunicator. 